[ayberk]

I have, but nothing stood out. I even tried Firefox Profiler (which is pretty good btw, kudos to the team), and it looks like DOM processing and renderer both spend a lot of CPU. But alas I'm not a frontend developer and have no incentive to dive deeper :)

[cassianoleal]

It could be specific websites. Google web apps are notoriously hostile to Firefox.

I leave Javascript off by default on uBO and only allow it selectively when I have something to gain on each website.